One of the most useful and lifesaving inventions of the 20th century is the smoke alarm. When I first learned about these alarms, I was surprised that many of them use a radioactive element to function. It was scary to think of this at first, but I learned about alpha particles, which smoke alarms use, and they can't penetrate a sheet of paper. The only way they would present a danger is if the container was ruptured and the particles become airborne. Then they can be inhaled.
The element used is Americium-241 in a very small amount, about 1/5000th of a gram.
Radioactive elements decay on a predictable basis and Americium releases an electron about 38,000 times per second. We'll get back to that.
Imagine a 9-volt battery with the positive terminal connected to a disc of metal; not far below this is a disc of metal with the negative terminal connected to this disc. There is a space between the discs and they do not touch. In this structure, virtually nothing is happening. If we put our small amount of Americium-121 below that bottom disc, we know it is releasing 38,000 electrons per second. These electrons disrupt the oxygen and hydrogen atoms of the air (all of our components are contained in an ionization chamber). The positive electrons of oxygen attach to the negative disc and the positive electrons of hydrogen attach to the negative disc. Like magnets, unlike particles attract. This creates a small electrical current that is monitored by the electronics of the smoke detector.
Particles of smoke disrupt this continuous flow of electrons, and set off an alarm. Relatively simple, no?
Another type of smoke detector is photoelectric, whereby a diode emits a ray of photons into an area with a photosensitive device around it. In normal mode, the photons do not strike the photocell, but rather, go in a straight line. If smoke interrupts the beam of photons, they will shift from a straight line and strike the photo sensing device, setting off the alarm.
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